Final-only agent UIs still need liveness and terminal reconciliation

When an agent UI intentionally suppresses commentary, commands, and tool events, preserve a neutral liveness signal (for example elapsed time or an activity pulse) and independently reconcile terminal state. Otherwise a slow healthy turn and a truncated or failed stream look identical. On clean EOF without a terminal event, treat the stream as incomplete and fetch authoritative session status; on errors, update both the displayed turn and session; while RUNNING, poll or reconnect with a bounded cadence. This applies to final-only candidate or customer surfaces; it does not justify exposing hidden reasoning or sensitive tool details.